The oscillation stability of a voltage controlled oscillators (VCO) may be perturbed by a number of aggressors present in their operational vicinity. The oscillator operational contamination may typically be due to frequency pushing/pulling of an oscillator core. To differentiate between the two (pushing and pulling), oscillator pushing may be categorized to be a change in the oscillator caused by changes in a supply voltage and/or electrical ground. Such artifacts present in the supply voltage or electrical ground may be caused by a number of factors, including but not limited to: a current rush caused by other circuits operating in a general vicinity, electrical ground contamination due to wiring and bonding resistances and parasitics, and so forth. On the contrary, oscillator pulling is due to the factors that impact the normal operation of the oscillator including but not limited to the interactions of the DCO core with its radiofrequency harmonics, interactions with similar RF frequencies elsewhere in a transmitter (including RF output), e.g., injection locking, and the impact of aggressors impacting the self-oscillation and the quality factor of the oscillator core, e.g., by electrical, magnetic or electro-magnetic coupling, etc.
Pushing/pulling of oscillators may be true for oscillators that are controlled by either analog or digital techniques. For example, frequency pushing/pulling may be a problem in a digitally controlled oscillator (DCO) that implements a capacitance of a tank circuit with segmented banks of capacitors controlled by a digital engine, such as an all-digital PLL (ADPLL). Even oscillators without tuning may be subject to pushing/pulling since they may also be subject to the aggressors like tunable oscillators.
As an example, when a transmitter's output power changes, the power supply may not be able to filter out the sudden change in the supply voltage and as a result, the oscillator's frequency may jump instantaneously. This may be interpreted as an instantaneous change in VCO or DCO gain (KVCO or KDCO). If the ADPLL has sufficient tuning range, the PLL lock may not be lost and the ADPLL may eventually recover depending on a bandwidth of a loop filter in the ADPLL. However, the transmitter's error vector magnitude (EVM), root mean square (RMS) and peak phase error, as well as spectrum may be severely degraded while the ADPLL recovers.
FIG. 1 illustrates a prior art oscillator 100. The oscillator 100 includes a phase detector 105, a loop filter 110, an oscillator 115, a resonator 120, and a frequency divider 125. A reference signal, fREF, may be input to the phase detector 105. The phase detector 105 may generate a phase error that may be proportional to a phase difference between the reference signal and a radio frequency (RF) continuous wave signal. The phase error may be filtered by the loop filter 110 to produce a slowly varying frequency command signal. The frequency command signal may be provided to the oscillator 115, typically a VCO or a DCO, which produces an RF signal, fLO. The RF signal may be dependent on the frequency command signal. The oscillator 115 may make use of the resonator 120 that oscillates in a desired frequency band. The RF signal may also be provided to the frequency divider 125 that may be used to generate the RF continuous wave signal by dividing the RF signal by N, which may be used by the phase detector 105.
FIG. 2 illustrates the oscillator 100 along with sources of aggressors that may cause frequency pushing/pulling. Possible aggressors may include power supply and ground bounce, RF leakage from on-chip coils and circuits that may be located in a pre-power amplifier section of a transceiver, RF output leakage from the transceiver's output, RF leakage from power amplifiers and other components, and so forth. These aggressors may all affect the oscillator 100.
A wide range of techniques have been used in the past to help mitigate frequency pushing/pulling, including: isolating the oscillator by physically separating the oscillator from other circuitry, using very high Q oscillators, using series ferrite elements, using an offset synthesizer architecture, improving the decoupling between the supply voltage and electrical ground and the oscillator, reducing electrical ground bounce, and so forth.